What this mod does:
With this mod installed, Sims can get fatter by overeating, build more muscle mass by working out or get skinnier by doing cardio. It also avoids "snapping" back to standard values for Sims created with other mods such as slider hacks.
A Sim's weight and fitness (that's muscle mass, not muscle definition) are usually limited to -1 to +1 by the game, this mod extends the values to -2 and +2 (see attached image). It does not add or remove code, it just adjusts those values.